Copyright Statement

Abstract

This review uses proxies of past temperature and atmospheric CO2 composition
based on fossil leaves to illustrate the uncertainties in biologically based proxies of
past environments. Most leaf-based proxies are geographically local or genetically
restricted and therefore can be confounded by evolution, extinction, changes in
local environment or immigration of species. Stomatal frequency proxies illustrate
how genetically restricted proxies can be particularly vulnerable to evolutionary
change. High predictive power in the modern world resulting from the use of a
very narrow calibration cannot be confidently extrapolated into the past (the
Ginkgo paradox). Many foliar physiognomic proxies of climate are geographically
local and use traits that are more or less fixed for individual species. Such proxies
can therefore be confounded by floristic turnover and biome shifts in the region of
calibration. Uncertainty in proxies tends to be greater for more ancient fossils. I
present a set of questions that should be considered before using a proxy. Good
proxies should be relatively protected from environmental and genetic change,
particularly through having high information content and being founded on biomechanical
or biochemical principles. Some current and potential developments
are discussed, including those that involve more mechanistically sound proxies and
better use of multivariate approaches.